1. Field of the Invention
This invention relates to an improvement in an electrically heatable transparency, and in particular to an electrical lead arrangement to provide power to the bus bars of a heatable windshield.
2A. Technical Considerations
It has been known to pass electric current through a bus bar and transparent conductive coating to another bus bar on the surface of a transparency to raise its temperature. Generally, a source of electrical potential is connected to the coating by way of first and second bus bars each positioned along opposite sides of the transparency to be heated. The bus bars may be comprised of metallic foil strips but in the case of glass transparencies they preferably are comprised of a metallic-ceramic frit material fused onto a surface of the transparency. Electroconductive leads are positioned along the transparency surface to interconnect the bus bars to the electrical power source.
In fabricating these heatable transparencies, the bus bars leads must be electrically insulated from the electroconductive coating so as not to interfere with the desire flow of current from one bus bar through the coating to the opposite bus bar. This is generally accomplished by using a moveable protective covering such as a mask or tape on the surface of the transparency in the area of the lead prior to applying the coating. After coating, the covering is removed leaving an uncoated area for the bus bar leads. As an alternative, the entire surface can be coated and the coating subsequently deleted from the bus bar lead areas.
2B. Patents of Interest
U.S. Pat. No. 2,843,713 to Morgan teaches an electrically heated article with an electrically conductive coating on the surface of the article and a plurality of discontinuous bus bar sections along the edge of the coated surface to reduce the flow of current from the bus bar elements to adjacent portions of the coating.
U.S. Pat. Nos. 3,752,348 to Dickason et al.; 4,543,466, 4,668,270, and 4,743,741 to Ramus; 4,654,067 to Ramus et al.; 4,718,932 to Pharms; and 4,744,844 to Hurst teach a heatable windshield and methods for making same. The bus bars and electroconductive coating of the windshields are positioned on an interior surface within the windshield assembly. U.S. Pat. Nos. 4,543,466 and 4,744,844 disclose an electroconductive coating, bus bars, and lead members to the bus bars applied to the same surface of a glass sheet.
U.S. Pat. Nos. 4,078,107 and 4,128,448 to Bitterace et al. teach a window with a heating circuit and antistatic circuit and method for its preparation. The heating circuit of the window includes wire members embedded within a plastic ply.
U.S. Pat. No. 4,213,028 to Wolf teaches an electric heating device for vehicle windows including a pair of elongated electrically non-conductive carrier strips having an adhesive layer on both sides and secured in spaced apart relation to the glass surface by one of the adhesive layers. Foil strips with an adhesive layer extend between the carrier strips with their ends overlaying and adhered to the carrier strip. An electrical terminal overlays the carrier strip and ends of the foil strips to electrically interconnect the heating device members.
U.S. Pat. Nos. 4,323,946 to Truax and 4,590,535 to Mang teach static charge dissipation circuits including interconnected wire members extending through a plastic layer of the window.